DENSITY-DEPENDENT SEX-RATIO IN ECHINOMERMELLA-MATSI (NEMATODA), A PARASITE OF THE SEA-URCHIN STRONGYLOCENTROTUS-DROEBACHIENSIS

We investigated the adult sex ratio in 70 infrapopulations of the nema tode Echinomermella matsi, a parasite of the green sea urchin Strongyl ocentrotus droebachiensis. The adult sex ratio was skewed towards fema le dominance at low adult intensity and towards male dominance at high adult intensity. We hypothesize that this is due to differences betwe en the sexes in development and mortality rates, or that female recrui tment is density dependent. A model with differences between the sexes in developmental and mortality rates may develop the observed sex rat ios if the female developmental and mortality rates are several times that of the males. A large difference in developmental rates between t he sexes appears unreasonable because the developmental rate for both sexes is low, and the predicted low female life-expectancy is unlikely because the males appear to accumulate in infrapopulations as the fem ales age. Density dependence of female numbers is, however, supported by a significantly lower female recruitment in infrapopulations with o ld females. We also find that the mean male length is negatively relat ed to measures of crowding, thereby supporting the hypothesis that com petition is of importance in E. matsi infrapopulations. A female bias at low intensities of infection, a density dependence in female recrui tment and the taxonomic position of E. matsi indicate that sex may be environmentally determined in this nematode.